1. Field of the Invention
The present invention relates to image processing systems and more particularly to apparatus and methods for filtering low amplitude texture and noise from video images.
2. Description of the Prior Art
In the prior art there are many image processing systems. The following are systems representative of the prior art.
A paper entitled "Adaptive Prefiltering for NTSC Composite Color Television Interfield Coding" was presented in the June 1981 Picture Coding Symposium by Sawada el al which teaches a nonlinear filter for image data in which a smoothing coefficient is generated from a difference signal such that the smoothing coefficient is set to zero for differences equal to or greater than a predetermined threshold and the smoothing coefficient is not modified for difference values less than the predetermined threshold.
The method and apparatus shown in the paper present a significant problem in that the smoothing coefficient is reduced to zero when a difference value exceeds a predetermined threshold which causes significant discontinuities in the filtered signal.
U.S. Pat. No. 4,121,248 discusses a streak reduction system for forward looking infrared systems employing an analog circuit apparatus for sampling, holding, integrating and comparing delayed signals to reduce streaks.
The patent does not show the digital apparatus or method according to the present invention.
U.S. Pat. No. 4,298,895 describes a method of noise elimination in a video image by counting a number of white picture elements among a group of picture elements surrounding a picture element to be acted upon and modifying the picture element value, when the number of white picture elements in the surrounding area exceeds a predetermined level, to the majority color value such as white and setting the current picture element to the minority color value such as black when the number of white elements in the surrounding area is less than a predetermined level. Alternatively, the value of the current picture element is set to the majority color value when the proportion is greater than a first predetermined level and is set to the color value of a minority color when the number of majority color picture elements is less than a second predetermined value and unmodified when the number of majority color value picture elements in the surrounding area is between the first and second predetermined levels.
The noise elimination method of the patent does not determine a difference value between intensity levels of a current picture element and one or more preceding picture elements and a difference value between a current picture element and one or more succeeding picture elements. Further, the method of the current patent does not show the generation of a filtered value for the current picture element by adding intensity values of the current picture element to a constant times a sum of limited first and second difference values.
U.S. Pat. No. 4,268,864 shows an image enhancement system for television signal wherein a first fractional amplitude portion of a generated detail signal is amplified and subjected to severe coring and a resulting bipolar signal is combined with a second fractional amplitude portion of the generated detail signal to produce a resultant detail signal having a contour which reduces the width of the edging effect at contrast transitions associated with conventional enhancement systems and in which high frequency noise and other spurious signals are substantially reduced while retaining fine detail.
Although the patent generally teaches image enhancement employing comparison of fractional portions of the generated detail signal in a video image to filter noise, the patent does not relate to a nonlinear filter for filtering noise in a digital video image by comparing values of current and preceding and succeeding picture elements to generate difference signals which are then employed to filter noise from the video image by adjusting value of the current picture element.
U.S. Pat. No. 4,228,465 shows a linear convolution filter which filters a current pixel value based upon values of pixels in a surrounding matrix.
The patent does not teach a nonlinear filter such as is taught and claimed by the present invention.
U.S. Pat. No. 4,242,705 teaches a recursive noise reduction system for a real time television system which employs a method of generating a noise signal which is subtracted from an incoming video signal to produce a noise reduced output signal for display.
The patent does not teach a nonlinear filter which reduces noise and smooths video images by adding limited difference signals generated from preceding and succeeding picture elements in an image to an original value of the current picture element to produce an essentially noise free picture element value.